The present invention is directed to clutches and, more particularly, to a bi-directional overrunning clutch for controlling torque transmission to the primary drive shaft of a wheeled machine.
In recent years there has been a tremendous demand for adding motor propulsion to what had previously been manual propelled machines. One example of such a device is walk-behind power equipment. These devices were once solely push-propelled. However, recently most models have become self-propelled. Examples of such equipment are lawnmowers, lawn vacuums, snowblowers, trimmers, edgers, concrete and asphalt cutters and the like. A number of factors have driven the move to self-propulsion, such as a desire for larger equipment which require less effort and allow for more detailed control. Speed of operation is another factor that has driven the increase demand for self-propelled equipment. Current self-propulsion systems for walk-behind equipment generally fall into two categories, simple and complex.
In a simple type drive system, a drive shaft or belt is connected to a single drive axle which drives both wheels. Since the wheels ware on a common shaft, they rotate at the same rate. The principal deficiency with this type of device is that no differential rotation is permitted between the drive wheels. As a result, the machine is not efficient during cornering which requires the outer wheel to travel a greater distance than the inner wheel. Because the outer wheel must travel farther than the inner wheel in the same amount of time, the outer wheel must rotate faster than the inner wheel. When the outer and inner wheels are fixed to a common axle, however, this differential rotation is not permitted. The result is that either the inner wheel is driven faster or the outer wheel is driven slower than necessary. In either case, cornering the equipment requires one of the wheels to slip or skid. This results in premature wear of the wheel.
Difficulty with cornering and wheel slippage are two major disadvantages with using equipment having drive wheels fixed to a common axle. Additionally, effort by the operator must be provided to overcome the ground-engaging forces to allow one wheel to slip. Furthermore, wheel slippage can cause damage to the surfaces on which the equipment is operating, as well as accelerated tire wear. For instance, turning a lawnmower with this type of drive system damages the turf under the slipping wheel.
One solution to this problem is described in U.S. Pat. No. 6,209,697 which describes a one directional overrunning clutch which is mounted in the hub of a wheel of a conventional walk-behind device, such as a lawnmower.
Complex drive systems for self-propelled, walk-behind power equipment generally provide a differential between the pair of drive wheels. The differential permits independent or differential rotation of the drive wheels on an axle when the user corners.
Many differentials on the market today use some form of an overrunning clutch to transmit torque when needed to a driven shaft. One successful use of an overrunning clutch in an all terrain vehicle is disclosed in U.S. Pat. No. 5,036,939. In that patent, the vehicle incorporates overrunning clutches where the wheel hub mounts to the axle, thus allowing each wheel to independently disengage when required.
Another successful use of an overrunning clutch in a differential is disclosed in U.S. Pat. No. 5,971,123, commonly owned by the assignee of the present invention. That patent describes an innovative electromechanical bi-directional overrunning clutch differential which addressed many of the problems inherent in the prior drive systems. The bi-directional overrunning clutch differential utilized an electrically controlled coil to advance and/or retard a roll cage, thereby controlling the ability of the differential to engage and disengage depending on the operational state of the primary and secondary wheels. The bi-directional differential in U.S. Pat. No. 5,971,123 also describes a backdrive system. The backdrive system actively engages the secondary shafts in certain situations where extra traction is needed. For example, when the vehicle is driving down a slope the system engages the front wheels, which are the wheels with the better traction.
Conventional complex differentials and overrunning clutches are generally costly to manufacture and, thus, relegated to more expensive vehicles, such as cars and four wheel drive vehicles.
A need exists for a less complex and less expensive bi-directional overrunning clutch that can be used in various self-propelled machines and light duty vehicles, such as snowblowers, lawn mowers, golf carts, and concrete and asphalt cutters.
According to the present invention there is provided an overrunning clutch for controlling torque transmission to a pair of shaft segments of a primary drive axle in a wheeled machine. The overrunning clutch includes a pair of hubs each engaged with an interior end of a corresponding shaft segment of the primary drive axle. The overrunning clutch further includes a roller assembly having rollers arranged in two sets, each associated with one of the hubs, and a roll cage having a plurality of recesses, each recess including at least one roller. The roll cage defines a central opening in which the pair of hubs is received. The overrunning clutch also includes a cam surface which is located radially outward from the roller assembly. The cam surface, acting in conjunction with the hubs, is adapted to provide wedging engagement of the rollers between the cam surface and the hubs when the roll cage is rotated relative to the clutch housing (forward-engagement position). The overrunning clutch further includes at least one friction member which is in contact with the roll cage and the hub such that, during operation, the friction member generates friction forces between the roll cage and the hub which cause the roll cage to turn with the hub, thus placing the roll cage in the forward-engagement position.
According to one embodiment of the invention, the overrunning clutch includes a pair of covers mounted on opposite sides of the roll cage for concomitant rotation therewith. In this embodiment, the friction member includes a pair of elastic frictional members, such as O-rings, one mounted in compression between each hub and an adjacent cover such that the elastic member causes the roll cage to turn in combination with the hub. In another embodiment of the invention, the friction member is a wave spring located between the roll cage and the hub.
The foregoing and other features of the invention and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments, as illustrated in the accompanying figures. As will be realized, the invention is capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and the description are to be regarded as illustrative in nature, and not as restrictive.